This invention relates to a quadrature coupled controllable oscillator comprising a first and a second identical circuit module, the two circuit modules each comprising an astable multivibrator circuit, the first circuit module being coupled with the second circuit module and the second circuit module being cross coupled with the first circuit module, the oscillator comprising in each of the circuit modules a first and a second Voltage Controlled Current Source (VCCS).
The invention further relates to a communication arrangement.
Quadrature coupled controlled oscillators are very useful devices used in applications as wireless local area networks, optical fiber networks, mobile telephony, transceivers and many others.
The main requirements that these oscillators have to fulfill are low phase noise and large tuning range. These requirements impose the use of quadrature ring LC oscillators that can have a high quality factor Q that implicitly determines a low phase noise and a large tuning range.
Quadrature ring LC oscillators use two identical oscillator units that are coupled in order to obtain quadrature output oscillations. The oscillator units are astable multivibrators with inductors as their loads. The negative resistances that active devices, field effect transistors for example, present across their output terminals overcome the loss in the inductors so that the oscillation is self sustained.
Such a quadrature ring LC oscillator is disclosed in the paper SP 24.6: A 900 MHz CMOS LCxe2x80x94Oscillator with Quadrature Outputs presented at ISSCC in 1996, Session 24, pp.392-393.
In this known arrangement the two LC oscillator units are coupled using field effect transistors (FET). The inductors and the parasitic FET gate and drain capacitors form equivalent LC cells. The oscillation frequency is determined by the inductors inductance and the FET gate and drain junctions capacitances. The total current in the equivalent LC cells is the sum of the drain current of the astable multivibrator transistor and the drain current through the coupling FET. These two currents are phase-shifted with 90 degrees with respect each other. It should be pointed out here that the oscillation frequency is determined by technologically dependent parameters as the FET drain and gate capacitances and as a direct consequence the oscillation frequency is technology dependent as well. Furthermore, because of the 90 degrees phase shift in the currents the coupling coefficient between the oscillator units is relatively low and under certain conditions the oscillations cannot be synchronized in quadrature. The phase noise is still relatively high because the resonant circuit achieved by the inductor and the drain and gate capacitors, that are distributed, cannot have a high quality factor.
It is therefore an object of the present invention to provide a quadrature coupled controlled oscillator with an increased coupling coefficient and whose oscillation frequency is determined independently of the fabrication technology.
In accordance with the invention, this object is achieved in a device as described in the introductory paragraph, which is characterized in that each of the circuit modules has a resonator for determining the oscillation frequency of the astable multivibrator circuit comprised in that module, and that in each of the circuit modules each of the VCCS is coupled with a respective phase shifter for shifting the phase of a current supplied by the VCCS to the resonator comprised in that circuit module.
The phase shifters introduce a phase shift in the currents produced by the VCCS such that the currents through the VCCS and through an active device of the astable multivibrator circuit are substantially in phase, providing a maximum current through the resonator. As a direct consequence, the coupling coefficient between the two circuit modules increase, a better stability of the oscillation being obtained.
In a preferred embodiment the resonators are LC resonators and determine the oscillation frequency of the oscillator. Their L and C components have their inductance and capacitance much bigger than any other parasitic inductance and capacitance in the circuit and, as a matter of consequence, the oscillation frequency is determined independently of the technology. The resonators provide low impedance at their resonant frequency so that they must be supplied with the necessary currents to maintain a stable frequency of oscillation. The phase shifters and the VCCS achieve this goal. The resonators can be any type of LC resonators that are characterized in that the current through them attains it""s maximum value at the oscillation frequency. As a direct consequence, the resonators are realized as a parallel connection between an inductor and a capacitor (tank circuit), two mutually inductively coupled tank circuits or any other combination of inductive and capacitive elements that behaves like a resonator characterized in that the current through it at the resonant frequency is maximum.
The oscillation frequency can be controlled in various modes, depending on the type of the resonator components, for example: electrically, mechanically, thermally, optically.
The oscillator according to the invention has the advantage of a lower phase noise and as a matter of consequence, the overall signal to noise ratio is increased.
Illustratively, all the previously described stages are realized with transistors and LC tank resonators. In an embodiment all these transistors are implemented in CMOS technology.
It is another object of the present invention to provide a communication arrangement for communicating via a bi-directional communication channel, comprising the novel oscillator disclosed and as claimed herein for generating a periodic signal, a receiving module for generating an output signal (OUT1) from the periodic signal and a received signal (IN) received from the channel, further comprising an emission module for generating an emission signal (OUT) for emitting to the channel from the periodic signal and an input signal (IN1).
The claimed communication arrangement is characterized in that the oscillator is conceived to provide a periodic signal to be mixed with the input signal (IN) in the receiving module in order to obtain a lower frequency signal (OUT1).
The communication arrangement is characterized in that the oscillator is conceived to provide a periodic signal to be mixed with the input signal (IN1) in the emission module in order to obtain the signal (OUT).